Gas sensor and manufacturing method for the same

ABSTRACT

An air side cover is attached to a proximal end of a housing so as to confine an aerial atmosphere therein. A measured gas side cover is attached to a distal end of the housing so as to confine a measured gas atmosphere therein. A glass sealing material airtightly seals a clearance between an inner surface of an insulator and an outer surface of a sensing element. A contact interface of the glass sealing material protrudes toward a proximal end of the gas sensor compared with at least an adjacent portion of the remainder of the glass sealing material. By melting and hardening a glass pellet, the sensing element is airtightly fixed in the insulator.

RELATED APPLICATION

This is a division of my copending commonly assigned application Ser.No. 09/879,069 filed Jun. 13, 2001.

BACKGROUND OF THE INVENTION

The present invention relates to a gas sensor installed in an exhaustgas system of an internal combustion engine for a combustion control orelse. Furthermore, the present invention relates to a method formanufacturing this gas sensor.

According to a conventional gas sensor, a sensing element is insertedinto an insulator. The insulator is fixed in a housing. A measured gasside cover is attached to a distal end of the housing. An air side coveris attached to a proximal end of the housing. The clearance between theinsulator and the housing is airtightly sealed. Similarly, the clearancebetween the sensing element and the insulator is airtightly sealed.

Presence of such an airtight sealing makes it possible to separate aninside space of the gas sensor into an aerial atmosphere and a measuredgas atmosphere.

In general, the sensing element has a measured gas sensing electrodeexposed to a measured gas and a reference gas sensing electrode exposedto the air serving as a reference gas. The sensing element produces asensing signal representing a gas concentration in the measured gasbased on an ion current or an electric potential produced between theseelectrodes. Hence, measurement of gas concentration cannot be performedaccurately when separation between the aerial atmosphere and themeasured gas atmosphere is insufficient.

Conventionally, powdered material, such as talc, and a sealing glass arelayered between the sensing element and the insulator to airtightlyseparate the aerial atmosphere and the measured gas atmosphere.

For example, U.S. Pat. No. 5,602,325 discloses a plurality ofsolid-phase sintered glass layers and a plurality of steatite spacerlayers which are alternately stacked in a ceramic sensor holder. Aceramic main body surrounds the alternately stacked glass layers andspacer layers. The ceramic main body extends to an outside housing.Furthermore, thin solid-phase sintered glass layers are interposedbetween the ceramic main body and each spacer layer.

Furthermore, U.S. Pat. Nos. 5,467,636 and 5,739,414 disclose a glasslayer interposed between a first ceramic insulating body and a secondceramic insulating body. According to this prior art, the glass issubjected to a compressive stress acting in the radial direction (i.e.,in the central direction) within an operating temperature zone.

However, securing airtightness by filling the powdered material, such astalc, requires checking many items to administrate the pressure and thefilling amount of the powdered material. This in disadvantageous incosts.

Furthermore, the glass layer is generally formed through the processesof placing the powdered glass material to a predetermined position,heating the powdered glass material to melt it, and then cooling themolten glass until it is hardened. This makes it difficult to obtain ahighly densified glass sealing material. Accordingly, it is difficult tomaintain satisfactory airtightness for a gas sensor based on a sealingarrangement using the glass sealing material only.

SUMMARY OF THE INVENTION

In view of the above-described conventional problems, an object of thepresent invention is to provide a gas sensor having an arrangementcapable of sealing the clearance between the insulator and the sensingelement with the glass material only. Furthermore, the present inventionprovides a manufacturing method for this sensor.

In order to accomplish the above and other related objects, the presentinvention provides a gas sensor comprising a cylindrical insulator, asensing element airtightly fixed in the insulator, and a cylindricalhousing having an inside space for placing the insulator, with an airside cover attached to a proximal end of this housing so as to confinean aerial atmosphere therein, wherein a glass sealing material seals aclearance between an inner surface of the insulator and an outer surfaceof the sensing element, and a proximal end surface of the glass sealingmaterial protrudes toward a proximal end of the gas sensor at a contactinterface of the glass sealing material to the inner surface of theinsulator and to the outer surface of the sensing element compared withat least an adjacent portion of the remainder.

The present invention is characterized in that the glass sealingmaterial seals a clearance between the inner surface of the insulatorand the outer surface of the sensing element. The proximal end surfaceof the glass sealing material protrudes toward the proximal end of thegas sensor at the contact interface of the glass sealing material to theinner surface of the insulator and to the outer surface of the sensingelement compared with at least an adjacent portion of the remainder.

Next, function of the present invention will be explained.

According to the present invention, the proximal end surface of theglass sealing material protrudes toward the proximal end of the gassensor at the contact interface of the glass sealing material to theinner surface of the insulator and to the outer surface of the sensingelement compared with at least an adjacent portion of the remainder(refer to FIG. 2). This arrangement makes it possible to firmly fix theglass sealing material to the sensing element and to the insulator atthe contact interface thereof, thereby maintaining improvedairtightness.

Accordingly, it becomes possible to surely provide an airtight sealingfor the clearance between the sensing element and the insulator by usinga single glass sealing material such as a glass pellet, i.e., withoutusing powdered material, and without requiring multistage fillingprocesses of the sealing material, and further without requiringcomplicated check of numerous managing items.

According to the present invention, it becomes possible to provide a gassensor capable of sealing the clearance between the insulator and thesensing element with the glass material only.

The glass sealing material is, for example, a material whose compositionis expressed by B₂O₃—ZnO—SiO₂—Al₂O₃—BaO—MgO.

This material has an excellent sealing ability for the sensing elementand the insulator. Thus, it becomes possible to ensure the reliableairtight sealing between the glass sealing material and the sensingelement as well as between the glass sealing material and the insulator.

Furthermore, the present invention is applicable to a gas sensorincorporating a cup-shaped solid electrolytic sensing element as shownin FIG. 1, and also applicable to a gas sensor incorporating amultilayered sensing element.

Furthermore, the arrangement of the present invention is applicable toan oxygen sensor and to an air-fuel ratio sensor for an automotiveinternal combustion engine. Especially, when formed into a multilayeredtype, the arrangement of the present invention is preferably applicableto a NOx sensor, a CO sensor or the like.

Next, according to the present invention, it is preferable that aprotruding portion of the proximal end surface corresponds to at least98% of the contact interface which extends circumferentially along anentire periphery of the glass sealing material.

The expression “at least 98% of the contact interface” means that thecontact interface between the glass sealing material and the innersurface of the insulator and the contact interface between the glasssealing material and the outer surface of the sensing element protrudeby an amount of 98% or more in the circumferential direction.

When the protruding portion exceeds 98%, it becomes possible to surelyprovide an airtight sealing for the clearance between the sensingelement and the insulator by using a single glass sealing material.

If the protruding portion is less than 98%, gas leakage may occur.

Needless to say, it is most preferable that the proximal end surface ofthe glass sealing material protrudes toward the proximal end of the gassensor entirely along the circumferentially extending contact interface.

Next, the present invention provides a method for manufacturing a gassensor comprising a cylindrical insulator, a sensing element airtightlyfixed in the insulator, and a cylindrical housing having an inside spacefor placing the insulator, with an air side cover attached to a proximalend of the housing so as to confine an aerial atmosphere therein and ameasured gas side cover attached to a distal end of the housing so as toconfine a measured gas atmosphere therein, wherein a glass sealingmaterial seals a clearance between an inner surface of the insulator andan outer surface of the sensing element, and a proximal end surface ofthe glass sealing material protrudes toward a proximal end of the gassensor at a contact interface of the glass sealing material to the innersurface of the insulator and to the outer surface of the sensing elementcompared with at least an adjacent portion of the remainder.

The method of the present invention comprises the steps of preparing acylindrical glass pellet having an outer shape fitting to the innersurface of the insulator and having a through-hole into which thesensing element is inserted, inserting the glass pellet into theinsulator and placing the sensing element in the through-hole of theglass pellet, and melting the glass pellet and then hardening the moltenglass to firmly seal the clearance between the inner surface of theinsulator and the outer surface of the sensing element.

According to the manufacturing method of the present invention, theglass pellet configured into a predetermined shape is inserted into theinsulator. Then, the sensing element is disposed in the through-hole ofthe glass pellet. Thereafter, the glass pellet is melted and hardened tofirmly seal the clearance between the insulator and the sensing element.

Accordingly, compared with a conventional method for directly fillingthe clearance with powdered glass material etc. as a glass sealingmaterial, it becomes possible to realize a highly densified glasssealing. Accordingly, it becomes easy to obtain a desired sealing inlength as well as in volume, thereby realizing a reliable sealing.

Accordingly, it becomes possible to firmly fix the sensing element andthe insulator at their interfaces so as to maintain excellentairtightness. Furthermore, it becomes possible to surely provide anairtight sealing for the clearance between the sensing element and theinsulator by using a single glass sealing material only.

As described above, the present invention makes it possible to provide amanufacturing method for a gas sensor capable of sealing the clearancebetween the insulator and the sensing element with the glass materialonly.

Regarding the shape of the glass pellet, it is possible to form theglass pellet with side surfaces fitting to the inner surface of theinsulator and to the outer surface of the sensing element. It is alsopossible to configure the glass pellet to have the through-holebeforehand so that the sensing element can be inserted into thisthrough-hole.

It is also possible to use the glass pellet consisting of a plurality ofparts.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription which is to be read in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a vertical cross-sectional diagram showing a gas senor inaccordance with a preferred embodiment of the present invention;

FIG. 2 is an enlarged cross-sectional diagram showing an essentialarrangement of the gas sensor in accordance with the preferredembodiment of the present invention;

FIG. 3 is a diagram showing the assembling of a sensing element, aninsulator, and a glass pellet in accordance with the preferredembodiment of the present invention;

FIG. 4A is an enlarged cross-sectional diagram showing a contactinterface between an inner surface of the insulator and a glass sealingmaterial in accordance with the preferred embodiment of the presentinvention;

FIG. 4B is a graph showing a raised amount of the contact interfacebetween the inner surface of the insulator and the glass sealingmaterial in relation to gas leakage amount in accordance with thepreferred embodiment of the present invention; and

FIG. 5 is a diagram showing an apparatus measuring the gas leakageamount of a tested gas sensor in accordance with the preferredembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be explainedhereinafter with reference to attached drawings. Identical parts aredenoted by the same reference numerals throughout the drawings.

Hereinafter, a gas sensor according to a preferred embodiment of thepresent invention will be explained with reference to FIGS. 1 to 5.

In this explanation, a front side of a gas sensor to be exposed to ameasured gas is referred to a distal end side and the opposite side isreferred to a proximal end side.

As shown in FIG. 1, a gas sensor 1 of this embodiment comprises acylindrical insulator 21, a sensing element 15 airtightly fixed in theinsulator 21, and a cylindrical housing 10 having an inside space forplacing the insulator 21. An air side cover 12 is attached to a proximalend of the housing 10 so as to confine an aerial atmosphere 142 therein.A measured gas side cover 13 is attached to a distal end of the housing10 so as to confine a measured gas atmosphere 141 therein.

As shown in FIG. 2, a glass sealing material 25 airtightly seals aclearance between an inner surface 210 of the insulator 21 and an outersurface 150 of the sensing element 15. A proximal end surface 255 of theglass sealing material 25 protrudes toward a proximal end of the gassensor 1 at a contact interface 250 of the glass sealing material 25 tothe inner surface 210 of the insulator 21 and at a contact interface 250of the glass sealing material 25 to the outer surface 150 of the sensingelement 15 compared with at least an adjacent portion of the remainder.

Hereinafter, this embodiment will be explained in more detail.

The gas sensor 1 of this embodiment is installed in an exhaust system ofan automotive internal combustion engine and is used for an air-fuelratio control of the internal combustion engine.

As shown in FIG. 1, in the gas sensor 1, the measured gas side cover 13attached at the distal end of the housing 10 consists of an outer cover131 and an inner cover 132 cooperatively constituting a double-layerconstruction. Both of the outer cover 131 and the inner cover 132 areprovided with holes 130 through which the measured gas is introducedinto the measured gas side cover 13 so as to form the measured gasatmosphere 141.

The air side cover 12 is provided at the proximal end of the housing 10.An outer cover 121 is overlapped with an outer surface of the air sidecover 12 at a proximal end thereof via a water-repellent filter 122. Theoverlapped portions of the air side cover 12 and the outer cover 121 areprovided with holes 120 for introducing air into the air side cover 12via the water-repellent filter 122.

The air side cover 12 has a smaller-diameter portion at its proximal endand a larger-diameter portion at its distal end which are integrally andcontinuously formed via a stepped portion 129.

The air introduced in the air side cover 12 through the air-introducingholes 120 forms the aerial atmosphere 142 of the gas sensor 1.

As shown in FIGS. 1 and 2, the housing 10 is configured into acylindrical shape and has two protrusions 101 and 102 protrudingradially inward from an inner surface thereof.

The protrusion 101, positioned at the proximal end side, has a receivingsurface 103 which supports a tapered portion 211 provided at an outersurface of the insulator 21.

The insulator 21 is made of alumina ceramic having fineness of 98%.

The tapered portion 211 is supported on the receiving surface 103 via anannular metallic packing 11 (refer to FIG. 2). The metallic packing 11is made of a nickel member having fineness of 99%.

The inside space of the gas sensor 1 is airtightly separated into theaerial atmosphere and the measured gas atmosphere at the portion wherethe metallic packing 11 is disposed.

An air side insulator 22 is disposed at a proximal end of the insulator21. An annular disc spring 220 is disposed between the air sideinsulator 22 and the stepped portion 129 of the air side cover 12.

A total of four leads 16 are disposed in an inside space of the air sideinsulator 22 so as to be electrically conductive with the sensingelement 15.

The sensing element 15, used for detecting an oxygen concentration, hasa multilayer body equipped with a built-in heater. Although not shown inthe drawing, the sensing element 15 has two sensor electrodes for takingout a sensor output signal, two power electrodes for supplying electricpower to the heater, and a total of four electrode terminals taken outof the sensor body.

The four leads 16 are disposed so as to be brought into contact withthese four electrode terminals respectively.

A proximal end of each lead 16 is connected to a lead 18 via a connector17 at an outside of the air side insulator 22. The lead 18 extends outof the gas sensor 1 through an elastic insulating member 23 disposed ata proximal end side of the air side cover 12.

As shown in FIG. 2, the sensing element 15 is placed in an inside spaceof the insulator 21. The glass sealing material 25 airtightly seals theclearance between the sensing element 15 and the insulator 21. Aproximal end surface 255 of the glass sealing material 25 is raised at acircumferential edge of the glass sealing material 25, i.e., at thecontact interface 250 to the sensing element 15 and to the insulator 21.

The contact interface 250 is annular. More specifically, the annularcontact interface 250 between the glass sealing material 25 and theinsulator 21 has a circular cross section. The annular contact interface250 between the glass sealing material 25 and the sensing element 15 hasa polygonal cross section. According to this embodiment, both of thecircular contact interface 250 and the polygonal contact interface 250are entirely raised along their circumferential peripheries.

Furthermore, the glass sealing material 25 contains 21% (weightpercentage) B₂O₃, 34.6% ZnO, 12.2% SiO₂, 4.9% Al₂O₃, 14.2% BaO, and12.7% MgO.

According to this embodiment, seal fixation between the sensing element15 and the insulator 21 of the gas sensor 1 is performed in thefollowing manner.

As shown in FIG. 3, a cylindrical glass pellet 26 is prepared inaddition to the insulator 21 and the sensing element 15. The cylindricalglass pellet 26 has an outer shape fitting to the inner surface 210 ofthe insulator 21 and has a through-hole 260 into which the sensingelement 15 is inserted. A proximal end surface 269 of the glass pellet26 is flat.

First, the sensing element 15 is inserted into the glass pellet 26.Next, the glass pellet 26 is inserted into the insulator 21. The orderof assembling the sensing element 15, the glass pellet 26, and theinsulator 21 is not limited to the above-described one and therefore canbe inversed.

Thereafter, these three members are integrally heated in a furnace atthe temperature of 800° C. to 950° C. for 30 minutes to five hours tomelt the glass pellet 26 and then naturally cooled down to harden themolten glass.

When the glass pellet 26 melts, the molten glass can be raised at thecontact interface 250 to the sensing element 15 and to the insulator 21due to surface tension. Therefore, after being naturally cooled down,the glass material protrudes at the contact interface 250 toward theproximal end of the gas sensor 1 while the remainder of the glassmaterial remains substantially flat as shown in FIGS. 1 and 2. Thus, theclearance is airtightly filled with the glass material so as to provideimproved sealing.

Then, the integrated assembly of the sensing, element 15 and theinsulator 21 is placed in the housing, 10 via a metallic packing 11,thereby constituting the gas sensor 1.

Next, functions and effects of this embodiment will be explained.

According to this embodiment, as shown in FIG. 2, the proximal endsurface 255 of the glass sealing material 25 protrudes toward theproximal end of the gas sensor 1 at the contact interface 250 of theglass sealing material 25 to the inner surface 210 of the insulator 21and to the outer surface 150 of the sensing element 15 compared with atleast an adjacent portion of the remainder. Thus, the interface betweenthe glass sealing material 25 and the sensing element 15 as well as theinterface between the glass sealing material 25 and the insulator 21 arefirmly fixed so as to provide excellent airtightness.

Furthermore, according to the method of this embodiment, as shown inFIG. 3, the glass pellet 26 configured into a predetermined shape isinserted into the insulator 21. Then, the sensing element 15 is disposedin the through-hole 260 of the glass pellet 26. Thereafter, the glasspellet 26 is melted and hardened to firmly seal the clearance betweenthe insulator 21 and the sensing element 15.

Accordingly, compared with a conventional method for filling theclearance with powdered glass material etc. as a glass sealing material,it becomes possible to realize an excellent sealing using the highlydensified glass sealing material 25.

Accordingly, it becomes possible to surely provide an airtight sealingfor the clearance between the sensing element and the insulator by usinga single glass sealing material such as a glass pellet, i.e., withoutusing powdered material, and without requiring multistage fillingprocesses of the sealing material, and further without requiring checkof numerous managing items.

According to this embodiment, it becomes possible to provide a gassensor capable of sealing the clearance between the insulator and thesensing element with the glass material only. Furthermore, it becomespossible to provide a method for manufacturing the gas sensor.

The following tables 1 to 6 show the components of other glass sealingmaterials preferable used for the gas sensor in accordance with thepresent invention. In each table, the content (wt %) represents a valueexpressed in terms of oxide.

TABLE 1 component content (wt %) B₂O₃ 21.0 ± 3 ZnO 34.6 ± 3 SiO₂ 12.6 ±3 Al₂O₃  4.9 ± 3 BaO 14.2 ± 3 MgO 12.7 ± 2

TABLE 2 component content (wt %) B₂O₃ 21.0 ± 3 ZnO 32.0 ± 3 SiO₂ 19.0 ±3 BaO 12.0 ± 3 MgO 16.0 ± 3

TABLE 3 component content (wt %) B₂O₃ 24.0 ± 3 ZnO 45.0 ± 5 SiO₂ 14.0 ±3 BaO  7.5 ± 3 MgO  7.5 ± 3

TABLE 4 component content (wt %) B₂O₃ 24.3 ± 3 ZnO 57.5 ± 5 SiO₂ 11.0 ±3 BaO  7.5 ± 3

TABLE 5 component content (wt %) B₂O₃ 22.6 ± 3 ZnO 34.5 ± 5 SiO₂ 12.8 ±3 BaO 11.5 ± 3 MgO 18.6 ± 3

TABLE 6 component content (wt %) B₂O₃ 19.0 ± 3 ZnO 30.4 ± 5 SiO₂ 16.0 ±3 Al₂O₃  5.0 ± 3 BaO 20.0 ± 3 CaO  9.6 ± 3

To evaluate the present invention, a gas leakage amount was measured inrelation to a raised amount of the contact interface 250 between theinner surface 210 of the insulator and the glass sealing material 25.

More specifically, many gas sensors were prepared and classified intotwo groups according to the condition (i.e., raised or sunken conditionas shown in FIG. 4A) of the contact interface 250 between the innersurface 210 of the insulator 21 and the glass sealing material 25 ineach gas sensor.

The gas leakage amount was measured in the following manner.

Each gas sensor was installed in an apparatus shown in FIG. 5 to measurea gas leakage amount at the air side and at the measured gas side. Theapparatus shown in FIG. 5 comprises a gas leakage amount measuringdevice 72 equipped with a valve 71 controlling an air supply amount, agas sensor attachment jig 74, and a valve 73 provided in a pipeconnecting the gas leakage amount measuring device 72 and the gas sensorattachment jig 74.

Hereinafter, the measuring method will be explained in more detail.

First, the gas sensor 1 is installed in the sensor attachment jig 74.The air side and the measured gas side are airtightly separated. In thiscondition, both of the valves 71 and 73 are opened to supply air into anair reservoir 740 of the attachment jig 74. A rubber packing 741 isprovided to seal the clearance between the housing 10 of the gas sensor1 and the attachment jig 74.

If the sealing between the insulator 21 and the glass sealing member 25is insufficient, air will leak from the clearance between them as shownby the arrows in the drawing. The pressure in the air reservoir 740decreases with elapsed time.

Accordingly, this apparatus is used to supply a predetermined amount ofair (4 atm) to the air reservoir 740 and then to measure a pressure dropin the air reservoir 740 after the passage of 10 seconds. The gasleakage amount (cm³) can be known from the measured pressure drop. It ishowever noted that a preliminary test should be done beforehand toconfirm no presence of gas leakage from other portions.

FIG. 4B shows the result of measurement.

From FIG. 4B, it is understood that any gas leakage may occurs when theraised amount is reduced to 0.

The raised amount can be precisely measured based on observation of thecontact interface 250 on a scanning electron microscopic view. Themeasurement data of this embodiment are thus obtained through thescanning electron microscopic observation.

Although this embodiment discloses the measurement result for thecontact interface 250 between the insulator 21 and the glass sealingmaterial 25, similar result was obtained when the gas leakage amount wasmeasured for the contact interface 250 between the sensing element 15and the glass sealing material 25.

Considering the evaluation test result, it is preferable that aprotruding portion of the proximal end surface 255 of the glass sealingmaterial 25 extends in a circumferential region corresponding to atleast 98% of the contact interface which extends circumferentially alongan entire periphery of the glass sealing material 25.

This invention may be embodied in several forms without departing fromthe spirit of essential characteristics thereof. The present embodimentsas described are therefore intended to be only illustrative and notrestrictive, since the scope of the invention is defined by the appendedclaims rather than by the description preceding them. All changes thatfall within the metes and bounds of the claims, or equivalents of suchmetes and bounds, are therefore intended to be embraced by the claims.

What is claimed is:
 1. A method for manufacturing a gas sensorcomprising a cylindrical insulator, a sensing element airtightly fixedin said insulator, and a cylindrical housing having an inside space forplacing said insulator, with an air side cover attached to a proximalend of said housing so as to confine an aerial atmosphere therein and ameasured gas side cover attached to a distal end of said housing so asto confine a measured gas atmosphere therein, wherein a glass sealingmaterial seals a clearance between an inner surface of said insulatorand an outer surface of said sensing element, and a proximal end surfaceof said glass sealing material protrudes toward a proximal end of saidgas sensor at a contact interface of said glass sealing material to theinner surface of said insulator and to the outer surface of said sensingelement compared with at least an adjacent portion of the remainder,said method comprising the steps of: preparing a cylindrical glasspellet having an outer shape fitting to said inner surface of saidinsulator and having a through-hole into which said sensing element isinserted, inserting said glass pellet into said insulator and placingsaid sensing element in said through-hole of said glass pellet, andmelting said glass pellet and then hardening the molten glass to firmlyseal the clearance between the inner surface of said insulator and theouter surface of said sensing element.